At present, the cone beam CT has been widely applied to stomatology and tumor radiotherapy already, which has the advantages of open structure and convenience in use, but compared with the sector beam CT, the cone beam CT has the disadvantage of inaccurate image density information.
When projection images are acquired by the cone beam CT, scattered photons influence the projection images, which is the main reason why the image density of the sector beam CT is inaccurate. At present, there are mainly two kinds of methods to reduce the influence of the scattered photons. One kind is a physical method. For example, a beam limitation device is adopted to limit the range of cone beams. Since the smaller the cone angle of the cone beam CT is, the fewer the components of the scattered photons in the projection images are, but while the range of the cone beams is limited, the imaging range is also limited. This method cannot be applicable to a larger imaging object. The other physical method is to add a backscattering grid between the imaging object and the flat panel detector. Although the backscattering grid can restrain the scattered photons in the projection images, noise may be introduced. This method has a better effect for the situation where the distance between the imaging object and the detector is smaller, but for the situation where the distance between the imaging object and the detector is larger, such as the cone beam CT which is integrated into an accelerator and is used in the image-guided radiotherapy, the effect of adding the backscattering grid is limited. The other kind is a method of postprocessing after collection of projection images. For example, a monte carlo algorithm is adopted, so that the distribution of the scattered photons in the projection images can be estimated accurately. Therefore, the influence of the scattered photons is eliminated from the projection images. However, even some simplified calculation technologies are adopted, the monte carlo algorithm has overlarge amount of calculation, and thus the scattering distribution with high resolution cannot generate within the time of clinical acceptability. The distribution of the scattered photons is calculated using an analytical model. Although the calculation speed is faster, for a complex imaging objective, a larger calculation deviation may be generated. For another example, a noise suppression reconstruction algorithm is adopted, and the scattered photons in the projection images are regarded as noise during image reconstruction, so that the influence of the scattered photons can also be reduced to a certain extent.
The above-mentioned technologies have effect of reducing the influence of the scattered photons, but also have limitations. At present, the accuracy of the reconstructed image density of the cone beam CT is still obviously lower than that of the traditional sector beam CT.
In consideration that the smaller the cone angle of the cone beam CT is, the fewer the components of the scattered photons in the projection images are, a rotating-grating cone beam CT imaging apparatus is invented. A grating limits X-rays emitted from the X-ray source to a plurality of narrow-angle cone beams or sector beams, and then the positions of the narrow-angle cone beams or sector beams are changed through the rotation of the grating, so that projection images of different regions of the imaging object are acquired, thereby greatly reducing the influence of the scattered photons on the image quality.